The present invention relates to a mixing device, and more especially to a device for homogeneously mixing and conveying a mixture, wherein the device is rotatable about its longitudinal axis and is provided with conveying elements distributed over its circumference. The invention also pertains to a method for the manufacture of a device of this type.
The device according to the invention is particularly adapted for use in electrophotographic copying machines, where it is employed for applying a pulverulent mixture composed of toner and carrier particles to a support material carrying a latent image. In this developing procedure, the toner particles are deposited in the electrically charged areas of the image, and following transfer to an image carrier they are fixed by fusing in a fixing station to render the image resistant to wiping.
The carrier particles do not adhere to the latent image and therefore are returned to a developer bin wherein a fresh developer mixture is prepared by replenishing the carrier with toner particles.
Various devices are known for applying the developer mixture in the above described manner and for simultaneously mixing the toner and carrier particles in a developer bin. For example, DE-OS No. 2,353,229 describes a circulating mechanism for a developer for electrostatic latent images, in which two transport rolls having blades mounted on their circumferences are disposed in a developer bin and rotate in opposite directions with respect to one another, in such a manner that the developer is, on the one hand, transported forward and backward in the longitudinal direction of the rolls and, on the other hand, is moved toward the recording medium by the roll which is disposed close to this medium. The blades of the transport rolls are arranged spirally over the circumferences of the rolls, thereby forming screws the pitch of which is variable in the direction of transport. This mechanism is expensive, because two transport rolls are used, and, above all, care has to be taken to ensure that the last and the first blades on each roll are in a particular position relative to the roll axles to avoid the formation of an increased pressure on the wall of the developer bin.
From DE-OS No. 2,437,389 a device is known for the uniform distribution of a developer employed in an electrophotographic copying machine. In this device a shaft is provided which carries a plurality of elliptical plates rotatable with the shaft and inclined relative to the axial direction of the shaft. When attaching the plates to the shaft, care has to be taken that the desired inclinations of the plates with respect to the axis of the shaft are adhered to. This design is particularly expensive in the event that the plates are individually welded to the shaft or are fastened by other means.
In U.S. Pat. No. 3,088,386 a mixing and distributing device is disclosed comprising two screws with blades. The two screws are identically designed, and they convey the developer mixture in opposite directions by rotating in opposite directions with respect to one another.
Similarly, DE-OS NO. 2,642,681 describes a device for mixing and feeding developer to a magnetic brush in an electrophotographic apparatus. The device consists of two screw conveyors arranged in parallel with the magnetic brush. These conveyors transport the developer in opposite directions and are connected at their end sections via reversing chambers. The two screw conveyors are positioned on the same level, lower than the magnetic brush roll. The excess developer trickling down from the magnetic brush drops back into one of the conveyor screws.
It is a disadvantage of the two last-mentioned devices that two screw conveyors are used, so that the space required is relatively large and, in addition, adequate arrangements must be made in order to prevent the developer mixture from piling up between the screw conveyors.
Japanese utility model application No. 6645/70 illustrates a device comprising a screw conveyor made up of two screws having opposite transport directions relative to one another. In this device, an accumulation of developer mixture may occur in the middle of the screw conveyor, thus causing a depletion of developer mixture at the ends of the conveyor. An accumulation of developer will also result in non-uniform mixing and irregular conveyance in the axial and radial directions.
In all of the known devices, no measures are taken to impart to them an adequate stiffness under flexure, even when these devices are used for large developing widths, for example, widths exceeding 300 mm.